oharag
Mechanical
- Dec 16, 2002
- 73
Hello all.
I just joined a new company two weeks ago. I was asked to investigate an issue they are having with a machined part. The part is a .25 dia. shaft that as a slot machined/grinded at the end of the shaft (think tuning fork). The material is 440C Stainless Steel. The manufacture making these shafts stated that they are machining the shafts in the annealed condition, and then hardening to our spec of 58 RHC (min). The reason for the hardness is due to the fact that the shaft is inserted into a friction clutch bearing. Turn the shaft in one direction with minimal effort, in the other direction the clutch friction locks requiring a force to overcome the friction. A pin (that is connected to a knob) is inserted into the slot to provide mechanical input for rotation of the machined shaft. The slot was speced as a full radius slot, but somewhere along the line a slot that has a flat bottom was also accepted.
Well the slots with the flat bottom came in and they started to fail. The failure was fracture related breaking right at the bottom of the flat slot. These shafts broke during normal operation. We attributed these failures to two things: brittleness of the hardened 440C shaft, and the stress concentrator of the sharp corners of the flat slot. We had discussions with the clutch manufacture about going with a softer material to reduce the chances of fracture, and we also recommended to the manufacture of the shaft to provide only shafts with a full radius at the bottom. The clutch manufacturer stated that it was okay to go a little lower on hardness (40 - 45 RHC) because we are not using the shaft any where near the rated limits of the clutch.
My question to you guys is: Could someone give me a detailed description/web site that shows the process/impact of the above operations? What I mean is what happens when the shaft is machined/ground? Do induced stresses incur at the corners of the flat shaft? I spoke with a heat treater, and he stated that the hardening process doesn't necessarily removed residual stresses from machining/grinding operations. Could there be microcracks formed at the corners of the flat slot? How does the hardening process impact the strength of the shaft near the slot? Does hardening induce additional stresses within the shaft? Is there a better process for the above (ie. heating to a lower temp to reduce residual stresses, and then hardening)? Is there an excellent resource on the internet to show how this whole process of hardening works (ie. What is annealing, hardening and tempering? How do they relate to one another?)?
It's been a while since getting out of college
I remember vaguely my fundamentals of material properties and metal machining classes. I can not locate these books. I searched the web for info. I just can not piece together the entire picture so that I can understand fully what is occurring here. I appreciate the help.
oharag
PS I'm also placing this post in the Metal and Metallurgy engineering forum because I believe it's related to that field as well.
I just joined a new company two weeks ago. I was asked to investigate an issue they are having with a machined part. The part is a .25 dia. shaft that as a slot machined/grinded at the end of the shaft (think tuning fork). The material is 440C Stainless Steel. The manufacture making these shafts stated that they are machining the shafts in the annealed condition, and then hardening to our spec of 58 RHC (min). The reason for the hardness is due to the fact that the shaft is inserted into a friction clutch bearing. Turn the shaft in one direction with minimal effort, in the other direction the clutch friction locks requiring a force to overcome the friction. A pin (that is connected to a knob) is inserted into the slot to provide mechanical input for rotation of the machined shaft. The slot was speced as a full radius slot, but somewhere along the line a slot that has a flat bottom was also accepted.
Well the slots with the flat bottom came in and they started to fail. The failure was fracture related breaking right at the bottom of the flat slot. These shafts broke during normal operation. We attributed these failures to two things: brittleness of the hardened 440C shaft, and the stress concentrator of the sharp corners of the flat slot. We had discussions with the clutch manufacture about going with a softer material to reduce the chances of fracture, and we also recommended to the manufacture of the shaft to provide only shafts with a full radius at the bottom. The clutch manufacturer stated that it was okay to go a little lower on hardness (40 - 45 RHC) because we are not using the shaft any where near the rated limits of the clutch.
My question to you guys is: Could someone give me a detailed description/web site that shows the process/impact of the above operations? What I mean is what happens when the shaft is machined/ground? Do induced stresses incur at the corners of the flat shaft? I spoke with a heat treater, and he stated that the hardening process doesn't necessarily removed residual stresses from machining/grinding operations. Could there be microcracks formed at the corners of the flat slot? How does the hardening process impact the strength of the shaft near the slot? Does hardening induce additional stresses within the shaft? Is there a better process for the above (ie. heating to a lower temp to reduce residual stresses, and then hardening)? Is there an excellent resource on the internet to show how this whole process of hardening works (ie. What is annealing, hardening and tempering? How do they relate to one another?)?
It's been a while since getting out of college
I remember vaguely my fundamentals of material properties and metal machining classes. I can not locate these books. I searched the web for info. I just can not piece together the entire picture so that I can understand fully what is occurring here. I appreciate the help.oharag
PS I'm also placing this post in the Metal and Metallurgy engineering forum because I believe it's related to that field as well.